A conventional automatic vehicle transmission includes a shift lever positioned in a vehicle interior in easy reach of a driver of the vehicle. Movement of the shift lever enables the driver to manually select a desired transmission operating range, usually from park, reverse, neutral, drive, and first gear/low. This lever, which is referred to as a PRNDL lever in the art, is mechanically coupled to a shift valve of the transmission by a length of cable. Tension on the cable during actuation of the shift lever moves the shift valve to thereby enable the selected operating range.
An Electronic Transmission Range Select (ETRS) system may be used as an alternative to a mechanically-actuated PRNDL lever. An ETRS system dispenses with the cable in favor of transmitting electronic signals, either to the shift valve or to flow/pressure control solenoids depending on the design. ETRS systems thus enable by-wire range selection, which can help to reduce weight while providing other performance advantages.
As is well known in the art, transmission park systems typically engage a park gear using a park pawl. The park gear is connected to a transmission output member. Thus, whenever the driver shifts the transmission into park, the park pawl is moved into a park position and teeth or splines of the park pawl engage mating teeth or splines of the park gear to prevent rotation of the output member. When the driver shifts the transmission out of park, the park pawl is pulled out of the park position and the teeth or splines of the park pawl disengage from the mating teeth or splines of the park gear to enable rotation of the output member. Pull out of the park pawl from the park position may be achieved by pulling, pushing, or torqueing the park pawl to move it from the park position.
When the transmission of a vehicle with an ETRS system is shifted from the park operating range to another operating range, a delay in pull out of the park pawl from the park position may occur. The delay may be caused by a lack of sufficient force to move the park pawl from the park position due to insufficient hydraulic pressure, loading of the park pawl, for example when the vehicle is parked on a grade, or other vehicle and transmission conditions. In some circumstances, it may not be possible to remove the park pawl from the park position, even after a delay.